Bilingualism provides a valuable tool for understanding language processing and its underlying neurocognitive mechanisms. However, the vast majority of bilingual research has involved spoken languages. Speech-sign bilinguals or bimodal bilinguals use separate perceptual and motoric systems for each language. This separation of systems affords the unique possibility of producing and perceiving elements from two languages at the same time, a phenomenon called code-blending. In contrast, speech-speech or unimodal bilinguals are restricted to producing one lexical item at a time because both languages rely on the same output channel (the vocal tract). This project investigates the implications of bimodal bilingualism for models of language production, for how bilingualism affects cognition, and for the brain's ability to manage and adapt to two language systems. In a series of psycholinguistic experiments, we investigate the consequences of dual- language activation for fluent and failed lexical access and explore processing effects specific to bimodal bilinguals (e.g., code-bends, sign language iconicity, changes in co-speech gesture, and the interpretation of facial gestures). A second series of studies investigates how bilingualism leads to changes in cognitive ability. The need to continuously control two languages during speech processing has been linked to advantages in cognitive control for unimodal bilinguals. Bimodal bilinguals provide the opportunity to consider whether these advantages arise from knowledge of two linguistic systems or from competition for a single modality (speech). We examine whether bimodal bilinguals exhibit particular cognitive advantages in spatial working memory and cross-modal monitoring. A third series of studies investigates whether the psycholinguistic and cognitive effects that we observe in behavior give rise to specific changes in the functional and structural architecture of the brain in both unimodal and bimodal bilinguals and whether these neural changes are tied to early brain development. Together these studies will characterize properties unique to bimodal bilinguals, while also revealing how bilinguals manage the activation of two languages in a single cognitive system. More generally, the results will enhance our understanding of the linguistic, cognitive, and neural systems that support language processing in all speakers. PUBLIC HEALTH RELEVANCE: With the spread of bilingual approaches to deaf education and the passage of the Americans with Disabilities Act (which has resulted a great need for sign language interpreters), more and more people are becoming bilingual in American Sign Language (ASL) and English. This project documents the linguistic, cognitive, and neural consequences of such bimodal bilingualism. The findings will help clinicians gain a more complete and accurate assessment of cognition and language in these bilinguals, will inform education and health matters related to bilingualism in general, and will be important in designing appropriate interpreter training programs, which will benefit deaf individuals and their families.